It is known that polymers having a main chain terminated in an alkenyl group as a crosslinkable functional group, when subjected to the reaction with a hydrosilyl group-containing compound as a curing agent or to photoreaction, is crosslinked and gives curing products excellent in heat resistance, durability and other properties. As examples of the main chain skeleton of such polymer having an alkenyl group-terminated main chain, there maybe mentioned, among others, polyether polymers such as polyethylene oxide; hydrocarbon polymers such as polyisobutylene, polybutadiene, polyisoprene and polychloroprene, and hydrogenated derivatives thereof; and polyester polymers such as polyethylene terephthalate, polybutylene terephthalate and polycaprolactone. These are used in various fields of application according to the main chain skeleton and the mode of crosslinking.
While these polymers are obtained by ionic polymerization or polycondensation, vinyl polymers obtained by radical polymerization and having a crosslinkable functional group-terminated main chain have scarcely been put to practical use. Among vinyl polymers, (meth)acrylic polymers have such characteristic features as high weathering resistance and transparency that the above-mentioned polyether polymers or polyester polymers cannot have. Thus, for example, (meth)acrylic polymers having an alkenyl group on side chains, not at either main chain terminus, are utilized in high weathering resistance coatings.
In contrast to such vinyl polymers having an alkenyl group on side chains, vinyl polymers having an alkenyl group-terminated main chain can give curing products with good curing characteristics. Therefore, a large number of researchers have endeavored to develop a simple and easy method of producing them. It is not easy, however, to produce them on an industrial scale. Japanese Kokai Publication Hei-04-132706 discloses a method of introducing a hydroxyl group by converting a halogen group at a polymer terminus. In crosslinking a polymer having a hydroxyl group-terminated main chain, however, it is necessary in many instances to use an isocyanate, which may pose a toxicity and/or stability problem. Hence, an alkenyl group or a crosslinkable silyl group is preferred as the crosslinkable terminal group.
Japanese Kokai Publication Hei-01-247403 discloses a method of synthesizing vinyl polymers having an alkenyl group at each of both termini which comprises using an alkenyl group-containing disulfide as a chain transfer agent. Further, Japanese Kokai Publication Hei-06-211922 discloses a method of synthesizing vinyl polymers having and alkenyl group at each of both termini which comprises synthesizing a vinyl polymer having a hydroxyl group at each of both termini using a hydroxyl-containing disulfide as a chain transfer agent and further introducing the alkenyl group into each terminus by utilizing the reactivity of the hydroxyl group. These methods, however, require the use of the chain transfer agent in large amounts for introducing the alkenyl group into both termini without fail, producing problems from the production process viewpoint. In addition, these methods employ the conventional radical polymerization, hence it is very difficult to control the molecular weight and molecular weight distribution (ratio between weight average molecular weight and number average molecular weight) of the polymer to be obtained.
Recently, the present inventors have succeeded in introducing various functional groups such as alkenyl, crosslinkable silyl and hydroxyl groups terminally into vinyl polymers by utilizing the technology of controlled radical polymerization, preferably living radical polymerization, more preferably atom transfer radical polymerization (see, for example, Japanese Kokai Publication Hei-11-080249, Japanese Kokai Publication Hei-11-080250, Japanese Kokai Publication Hei-11-005815, Japanese Kokai Publication Hei-11-116617, Japanese Kokai Publication Hei-11-116606, Japanese Kokai Publication Hei-11-080571, Japanese Kokai Publication Hei-11-080570, Japanese Kokai Publication Hei-11-130931, Japanese Kokai Publication Hei-11-100433, Japanese Kokai Publication Hei-11-116763, Japanese Kokai Publication Hei-09-272714 and Japanese Kokai Publication Hei-09-272715).
On the other hand, in cases where the polymer contains a halogen, the halogen may cause various problems in using this polymer in various fields of application. For example, it may corrode metals in contact therewith, or reduce the storage stability of curable compositions comprising the polymer. In cases where the polymer is a product obtained by atom transfer radical polymerization, for instance, the polymer terminus or termini have a halogen introduced therein and, therefore, such problems as mentioned above become more serious.
Meanwhile, WO 99/54365 discloses a method of eliminating a halogen group from polymer terminus as halogen-containing compound by utilizing addition and elimination reactions of a specific double bond-containing compound. By this method, however, an acidic compound such as a hydrogen halide will be produced. In addition, unexpected side reactions such as crosslinking reactions may possibly proceed.
In view of the above-mentioned state of the art, it is an object of the present invention to reduce the halogen content of a halogen-containing polymer and thus provide a polymer and a curable composition, without producing such problems as corrosion of metals in contact with the same or reduction in storage stability.